1. Field of the Invention
The present invention relates to a motor drive apparatus, an integrated circuit, and a motor drive method.
2. Description of the Related Art
A brushless motor is mainly used in air conditioner motors (a blower fan motor and the like) for automobiles or electric appliances because of great demand for lower noise. Brushless motors for these applications comprise an inexpensive hall device for detecting a relative position between a rotor and a stator.
A conventional motor drive apparatus, when driving a brushless motor apparatus having a hall device, controls conduction/non-conduction of drive transistors for switching current directions through each drive coil of the brushless motor based on a position detection signal detected by the hall device. By controlling the conduction timings of the drive transistors so as to be appropriate timings, the brushless motor is driven in an appropriate rotational direction.
Note that even if a brushless motor is used, when the rotation speed varies such as when the rotation speed is changed from low to high, a buzz occurs in rotation sound thereby causing noise (so-called buzzing). It is known that instead of rendering a predetermined drive transistor conductive at the input timing of the position detection signal detected by the hall device, by rendering the predetermined drive transistor conductive at a timing delayed from the input timing of the position detection signal detected by the hall device, noise associated with the variation in the rotation speed is reduced.
A conventional motor drive apparatus uses the slow waveform of the charge/discharge voltage of a capacitor in order to set time from the input timing of the position detection signal to the time the predetermined drive transistor is rendered conductive (hereinafter, referred to as “timing delay time TC”). The timing delay time TC is expressed by the approximate equation “TC=C×V÷I” where C, I, and V are the capacitance, charge/discharge current, and voltage of the capacitor respectively. Hence, conventionally the timing delay time TC is set by switching a plurality of (e.g., three) predetermined charge/discharge currents I according to the rotation speed setting voltage ratio VIND (%) for setting the rotation speed at a value specified by an external device such as a microcomputer.
For example, in FIG. 8, the timing delay time TC is switched between three levels: 0.7 ms for the rotation speed setting voltage ratio VIND (%) of 0 to 46%, 0.4 ms for the rotation speed setting voltage ratio VIND (%) of 47 to 66%, 0.1 ms for the rotation speed setting voltage ratio VIND (%) of 67 to 100%. See for example Japanese Patent Application Laid-open Publication No. 2002-325479.
If charge/discharge currents I are switched stepwise according to the rotation speed setting voltage ratio VIND (%), thus setting the timing delay time TC at a plurality of levels, the timing delay time TC changes sharply at the boundaries between the levels. Thus, the problem occurs that, when the rotation speed setting voltage ratio VIND (%) specified by the external device is close to one of the boundaries between the levels, or when the rotation speed setting voltage ratio VIND (%) passes through the boundaries between the levels while the rotation speed changes continuously according to rotation speed control by the external device, the setting of the timing delay time TC does not expectedly produce the effect of reducing noise.